Microelectronic devices, such as integrated circuit (IC) chips formed on a semiconductor substrate wafer, have grown increasing complex over the past several years. By miniaturizing the circuits of the microelectronic devices, industry has achieved significant performance improvements in terms of increased processing speed and decreased footprint. However, the miniaturized circuits are difficult to form. Minor contamination by impurities and other imperfections have greater and greater effects on the integrity of the devices as the size of circuits within microelectronic devices decrease. As industry transitions from the present 0.25 micron circuit devices to devices having smaller circuits, such as 0.18 and 0.13 microns, device formation techniques will have to provide greater precision using a wider variety of materials and with decreased contamination of the device. One example of a new material designed to reduce device size is the use of copper instead of aluminum to form device interconnects.
Microelectronic devices can be formed on substrates in a number of different ways. Some conventional techniques for forming microelectronic devices include rapid thermal processing (RTP), etch processing, and physical vapor deposition (PVD). PVD occurs in a relatively low pressure environment. A target, comprised of the material to be deposited, and the substrate are disposed in a reaction process chamber with a low pressure plasma gas. The target deposits the material on the substrate by the creation of an electric charge difference between the target and the substrate.
Chemical vapor deposition (CVD) is another example of a conventional and well-known process for depositing materials on a substrate to fabricate a microelectronic device on the substrate, such as in the fabrication of a semiconductor IC chip. To achieve a uniform growth of a thin-film material on a substrate, conventional CVD systems attempt to distribute a precursor gas, sometimes in combination with other reactant gases, in a uniform flow over the substrate. Under predetermined conditions for the precursor, such as predetermined temperature and pressure conditions within the CVD reaction process chamber and the substrate, the precursor deposits a desired material on the substrate as the precursor flows over the substrate. For instance, CVD provides excellent thin-film deposition of copper, tantalum nitride, titanium nitride, barium strontium titanate, and other materials typically used as thin-films for device fabrication on a substrate.
PVD and CVD provide different advantages based upon the material to be deposited. For example, CVD provides significant advantages in the deposition of a uniform thin-film of copper on a substrate. However, it is difficult to manufacture microelectronic devices by combining PVD and CVD processes due to the relatively high pressure of the process gas used in the reaction process chamber for CVD compared to the low pressure used for PVD. Further, the gases used to support CVD tend to damage substrates if the CVD gases are inadvertently introduced during a PVD process.
Typically, CVD occurs in a reaction process chamber that provides a low-conductance, contaminant-free environment for flowing the precursor over the substrate in a uniform manner. Alternatively, CVD can be performed in a high-conductance reaction process chamber that provides a relatively large flow of process gas to achieve a uniform film deposition. High-conductance systems generally have a larger footprint than do low-conductance systems, and use a greater amount of process gas for a given film deposition thickness. After deposition, the precursor is evacuated from the reaction process chamber to allow deposition of a subsequent material film, or to allow transfer of the substrate to another reaction process chamber for deposition of the subsequent material film. CVC, Inc. has a hub system that connects a number of reaction process chambers through a central hub to allow transfer of the substrate. The central hub is maintained at a low pressure to minimize the introduction of contaminants during transfer of substrate wafers through the hub.
Conventional single wafer CVD systems feed gases above and perpendicular to the substrate wafer. The gases deflect from the center of the wafer and flow radially from the center to an exhaust port located below the substrate wafer. In such conventional systems, the center of the substrate tends to receive a higher concentration of process chemicals associated with the gases, resulting in faster thin-film material growth at the center of the substrate than at the edges. This can lead to a bell-shaped film thickness with a thicker film at the center of the substrate than at the edge.
To alleviate this difficulty, conventional CVD systems use a showerhead arrangement. The precursor gas flows from above the showerhead into a centrally-located inlet of the showerhead housing. The showerhead housing has a showerhead gas dispersion plate with several hundred small openings to allow a low-conductance flow of the precursor gas to the CVD reaction chamber for more-uniform distribution across the substrate. To encourage a uniform distribution of the precursor gas from the dispersion plate openings, a deflector plate is typically disposed between the incoming gas flow and the dispersion plate. The deflector plate deflects the incoming gas flow radially from the intake vector to fill the showerhead housing with gas before the gas flows through the openings, thus avoiding an excessive concentration of gas flow over the center of the substrate.
Although a deflector plate and showerhead in a conventional CVD system can aid in the relatively uniform distribution of gas across the substrate, this arrangement creates a number of difficulties in the commercial production of microelectronic devices on a substrate wafer. For instance, the process gas inlet at the top of the showerhead increases the height footprint of the system and vertical thickness of the showerhead housing. This can increase the amount of precursor gas needed for deposition of a given film. Further, the inlet and associated fittings increase the difficulty of showerhead maintenance, and the likelihood of contamination during CVD processing. For example, to allow servicing of the showerhead, flexible hoses are often used between the showerhead inlet and process gas source. These hoses impede access to the showerhead housing, and can include particulate contaminates that can break free during CVD processing to introduce contaminants to the substrate.
Another difficulty associated with conventional CVD systems relates to system throughput. During CVD processing, gases are distributed from the showerhead inlet, through the dispersion plate and across the substrate with a low-conductance uniform flow. After deposition of the desired film, gas flow through the inlet is ceased by a shutoff valve, and residual gases are removed from reaction chamber through an exhaust located at the bottom of the reaction chamber. This results in process gas flowing over the entire length of the reaction chamber. Once the residual gas is removed from the reaction chamber, the substrate can be removed from the reaction chamber for further processing. For instance, the hub system sold by CVC, Inc. can move the substrate between several reaction chambers through a central hub, thus minimizing contamination of the substrate between the deposition of different material layers in separate reaction chambers.
To minimize contamination of the hub and associated reaction chambers during substrate handling, a thorough evacuation of residual gases upon completion of a deposition process is generally accomplished before transfer of the substrate through the hub. The low conductance of the reaction chamber and showerhead dispersion plate openings tends to increase the time needed to evacuate the reaction chamber since the evacuation pump has to draw residual process gas through the openings for evacuation of the showerhead housing. In low-conductance systems, baffles associated with the reaction chamber also impede evacuation of residual gas. Further, even with an extensive evacuation time, residual gas typically remains in the precursor delivery line, the showerhead housing and the reaction chamber, resulting in plating of material from the precursor on the wafer handling system, such as the wafer chuck, when the residual gas decomposes, and eventual contamination of the system. Increased evacuation time can decrease the presence of residual gas, but even extensive evacuation times generally cannot eliminate the residual gas from the showerhead and reaction chamber before transfer of the substrate wafer through the hub. The increased evacuation times lead to a corresponding decrease in system throughput.
Another difficulty of conventional CVD systems results from CVD processes that use two or more gases to deposit a material on a substrate. For instance, a precursor and reducing gas chemically support deposition of a material on a substrate, but are chemically incompatible if mixed before delivery to the substrate. If the precursor and reducing gas are mixed in the delivery line or showerhead housing before flowing to the reaction chamber, they will generate particles that cause blockage of the gas delivery system and that can cause undesired composition of the film material.
One conventional technique for delivery of plural gases without premixing is to use a multi-zone showerhead. The incompatible gases are fed into separate rings in the showerhead housing for delivery to the reaction chamber by separate concentric zones of dispersion plate openings. However, the multiple zones typically result in the deposited film having a ring pattern similar to the pattern of the zones of the dispersion plate. Multiple zones designed with smaller zones to minimize the ring-pattern of the deposited film also have an increased resistance to flow in each zone. The increased flow resistance decreases system throughput by increasing pumping and purging cycle times and can cause condensation of pressure-sensitive precursor vapor. Further, the multi-zone showerhead design is difficult to manufacture and inflexible with respect to its use with various combinations of gases, flow rates and reactor geometries.
Another difficulty associated with CVD relates to the deposition of the material from the precursor gas to the reaction chamber walls and to the chuck that supports the substrate in the reaction chamber. CVD of a copper film presents increased difficulty due to the narrow range of conditions in which the copper precursor is stable. For instance, one typical copper precursor will decompose at temperatures above 100 C, and will condense at temperatures below 50 C. Thus, over a series of CVD depositions, a reaction chamber and chuck used for copper deposition tends to have a residual film of copper build, which can interfere with subsequent depositions.